Reducing carbohydrates in your diet will cause your body to flush out water and therefore cause you to lose weight.
Gary Taubes writes,
In 1919, Francis Benedict, director of the Nutrition Laboratory of the Carnegie Institute of Washington, described it this way: "With diets predominantly carbohydrate there is a strong tendency for the body to retain water, while with diets predominantly fat there is a distinct tendency for the body to lose water." The context of Benedict's discussion was the weight loss that occurs in the first few weeks of any calorie- or carbohydrate-restricted diet, adn particularly the latter. As Benedict pointed out, thi sweight loss is to a large extent water, not fat, which has to be factored into any discussion of the apparent benefits of a reducing scheme. In the late 1950s, a new generation of investigators rediscovered the phenomenon, and it was then used to rationalize the popularity of carbohydrate-restricted diets as due not to the ease of losing fat, but entirely to the water lost in the first few weeks of the diet.
What do we glean from this? First, that if you wanted to drop a few pounds maybe to make weight for a wrestling match, boxing match, or some other event where scale weight matters, cut back on carbohydrates, and maybe on calories too. Reducing salt would also help since salt causes the body to retain water. Second, that the quick weight loss experienced by cutting calories or cutting carbs is mostly water weight, and therefore not getting us where we want to be (unless we're a wrestler or boxer) because we probably care more about how we look and how much fat our bodies retain, as opposed to how much we weigh on a scale.
Taubes goes on to say,
[T]he water lost on carbohydrate-restricted diets is caused by a reversal of the sodium retention that takes place routinely when we eat carbohydrates. Eating carbohydrates prompts the kidneys to hold on to salt, rather than excrete it. The body then retains extra water to keep the sodium concentration of the blood constant. So, rather than having water retention caused by taking in more sodium, which is what theoreticlaly happens when we eat more salt, carbohydrates cause us to retian water by inhibiting the excretion of the sodium that is already there. Removing carbohydrates from the diet works, in effect, just like the antihypertensive drugs known as diuretics, which cause the kidneys to excrete sodium, and water along with it.
I think this is an interesting piece of information that I have never heard--or at least not heard explained in this way. I think it is important to understand the above if one is to be able to defend a low-carb lifestyle, but also to be able to explain why people have so much success in the first few weeks of the Atkins diet or similar low carb diets, or even with calorie restricted diets.
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What I've noticed via self-experimentation/anecdotal evidence is that if I've been sufficiently glycogen-depleted and go on a carb-up*, my bodyweight can go up ten pounds in a matter of 48 hours. The only conceivable explanation for this is that replenishing glycogen stores means huge amounts of water retention. And it only goes to follow that a return to low-carbbing, usually by way of an intermittent fast, quickly returns me to the pre-carb-up weight.
When you understand that every pound of fat is 3,500 calories, then you appreciate that not only is it difficult to take off pounds of fat via caloric deficits (i.e. 500 calories a day deficit will only net you a pound of fat in a week), but it's also difficult to put on fat -- because even though you might occassionally overeat by 1-2,000 calories, sustained overeating like that is unlikely *and* your body is going to heat-cycle a great deal of those calories into energy (if nothing else, by way of adaptive thermogenesis).
Just a few thoughts.
* That's a nice way of saying carb-binge.